The Israeli military is reportedly using flechette shells in its offensive in Gaza, weapons described as illegal under “rules of humanitarian law” by an Israeli human rights organisation.

Generally fired by a tank and described as an “imprecise weapon” ill-suited to combat in a built-up area, flechette shells explode in the air above a target, sending out a cone of thousands of tiny steel darts each no more than 4cm (1.57 ins) long.

According to the Palestinian Centre for Human Rights (PCHR), six such shells were fired towards the village of Khuzaa in the Gaza Strip on 17 July, the Guardian reported.

The PCHR was described as saying that a woman, 37-year-old Nahla Khalil Najjar, suffered injuries to her chest in the flechette shelling, and pictures purporting to show the ammunition were passed on by a PCHR fieldworker.

When approached by the newspaper, the Israeli military did not deny using the shells in the conflict, saying that it only deploys weapons “determined lawful under international law”.

Flechette rounds have been used before in Gaza, and were declared legal by the Israeli supreme court in 2002. But a 2011 report by the Israeli human rights group B’Tselem described them as carrying “a particularly high danger of harming innocent civilians”.

“While flechettes are not expressly forbidden under international humanitarian law in all circumstances, other rules of humanitarian law render their use in the Gaza Strip illegal,” B’Tselem said.

“One of the most fundamental principles is the obligation to distinguish between those who are involved and those who are not involved in the fighting, and to avoid to the extent possible injury to those who are not involved.”